用成员函数重载实现is_base_of

std::is_base_of是一个C++ 11标准中用于判断某个类是否是另一个类父类的类模板。

#include "killCmake.h"

#include<string>

using namespace std;

class A
{
};

class B : public A
{
public:
	B(int x): x_(x)
	{}
private:
	int x_;
};

//template<typename Base,typename Derived>
//struct is_base_of {...};


int main()
{

	std::cout << std::is_base_of<A, A>::value << std::endl;
	std::cout << std::is_base_of<B, A>::value << std::endl;
	std::cout << std::is_base_of<A, B>::value << std::endl;

	return 0;
}

C++ 17标准中又引入了变量模板简化std::is_base_of的书写。

#include "killCmake.h"

#include<string>

using namespace std;

class A
{
};

class B : public A
{
public:
	B(int x): x_(x)
	{}
private:
	int x_;
};

//template<typename Base,typename Derived>
//struct is_base_of {...};

template<class Base,class Derived>
inline constexpr bool is_base_of_v_v = std::is_base_of<Base, Derived>::value;

int main()
{

	std::cout << std::is_base_of<A, A>::value << std::endl;
	std::cout << std::is_base_of<B, A>::value << std::endl;
	std::cout << std::is_base_of<A, B>::value << std::endl;

	std::cout << std::endl;
	// 简化版本
	std::cout << is_base_of_v_v<A, A> << std::endl;
	std::cout << is_base_of_v_v<B, A> << std::endl;
	std::cout << is_base_of_v_v<A, B> << std::endl;

	return 0;
}

std::is_base_of的实现代码，写一个IsBaseOf类模板来实现，代码如下。

#include "killCmake.h"

#include<string>

using namespace std;

class A
{
};

class B : public A
{
public:
	B(int x): x_(x)
	{}
private:
	int x_;
};

//template<typename Base,typename Derived>
//struct is_base_of {...};

template<class Base,class Derived>
inline constexpr bool is_base_of_v_v = std::is_base_of<Base, Derived>::value;

template<typename Base,typename Derived> // <父类，子类>
class IsBaseOf
{
private:
	template<typename T>
	static std::true_type test(T*);

	template<typename>
	static std::false_type test(void*);

	template<typename B,typename D>
	static auto test_middle() -> decltype(test<B>(static_cast<D*>(nullptr)));
	// 调用test()

public:
	static constexpr bool value = IsSameType < std::integral_constant<bool, std::is_class_v<Base>&& std::is_class_v<Derived>&& decltype(test_middle<Base, Derived>())::value
		, std::integral_constant<bool, true>>::value;
};


int main()
{

	std::cout << std::is_base_of<A, A>::value << std::endl;
	std::cout << std::is_base_of<B, A>::value << std::endl;
	std::cout << std::is_base_of<A, B>::value << std::endl;

	std::cout << std::endl;
	// 简化版本
	std::cout << is_base_of_v_v<A, A> << std::endl;
	std::cout << is_base_of_v_v<B, A> << std::endl;
	std::cout << is_base_of_v_v<A, B> << std::endl;

	return 0;
}

未完待续，干他菊花就对了